The present invention relates to an ignition system for internal combustion engines, and more particularly to an ignition system of the electronic distribution type for internal combustion engines, which is suitable for a multi-cylinder engine, such as a gasoline engine for automobiles, and in which an ignition plug is energized with a high voltage induced in a secondary coil when the current flowing through a primary coil of an ignition coil is cut off by a semiconductor switching device.
In multi-cylinder engines, such as gasoline engines for automobiles, by way of example, ignition systems of electronic distribution type have been recently introduced in which one ignition coil is provided for each one or pair of ignition plugs to perform ignition control without using a distributor.
Such an ignition system of the electronic distribution type requires a plurality of switching circuits which include power transistors and which carry out power supply control to conduct or cut off primary currents of the ignition coil, and a current detecting circuit which comprises a plurality of feedback control circuits for limiting currents so that the currents flowing through the ignition coils are controlled to a predetermined value, and resistance devices for detecting the currents. In this connection, attempts have been made at suppressing an increase in the cost by providing some control circuits and some circuit elements as shared devices by the use of plural ignition coils. One example of those attempts is disclosed in JP, A, 60-20667.
Also, JP, A, 3-43669 discloses a scheme in which it is proposed to achieve such common use for the same purpose. In this prior art arrangement, switching circuits and feedback control circuits are provided independently for all ignition coils, and only current detecting circuits are provided in common for every two successive circuits.
In the above-mentioned prior art arrangements, the feedback control circuits and the current detecting circuits are formed on a thick-film circuit board, and the ignition system is constituted in the form of a package. For this construction, a GND portion on the thick-film circuit board is connected to an external GND terminal by a single wire.
An ignition system of the electronic distribution type is used under thermally severe situations in the vicinity of an engine. Therefore, the wire used in the above-mentioned prior art arrangement to interconnect the GND portion on the thickfilm circuit board and the external GND terminal is formed into a predetermined shape to effectively absorb deformations due to repeated changes in temperature, and a nickel wire with a diameter of 0.3 mm is used in consideration of formability and elastic strength. The continuous withstand current of this wire is about 10 A. During operation of the ignition system, a current on the order of max. 8 A flows through the current detecting circuit. Accordingly, the wire of the GND line will not be disconnected by fusion and thus will function normally. However, it has been recently found that there are conditions in which the wire of the GND line may possibly be disconnected by fusion. Such a possibility is supposedly attributable to the following abnormal operation of the ignition system. Specifically, under certain conditions, the base current in the ignition system may be continuously kept in a HIGH state due to floating of the GND, noises, malfunction and other causes, which results in a condition of abnormal power supply wherein ignition coils for all cylinders cause ignition at the same time. In this event, the total current given by multiplying the current flowing through each current detecting resistance device by the number of the resistance devices flows through the wire of the GND line, causing the wire to be disconnected by fusion.
Furthermore, while the sequence of distribution is usually determined in ignition systems so as to ignite multiple cylinders one by one in sequence, two cylinders are simultaneously ignited in the limp-form traveling mode of operation. To surely start up the engine, a distribution method of simultaneously igniting two cylinders only at start-up may be employed in some cases. However, if such a mode of simultaneously igniting two cylinders is adopted in the above-mentioned prior systems, a current as large as twice the current flowing through the current detecting resistance device would flow through the wire of the GND line and the wire would be disconnected by fusion as with the foregoing event of abnormal operation. Accordingly, the mode of simultaneously igniting two cylinders cannot be adopted in the prior art.